Researchers with the Lawrence Berkeley National Laboratory and the University of California Berkeley have measured what is believed to be the smallest force yet recorded – 42 yoctonewtons, or a septillionth of a newton.

Measuring force at such incomprehensibly tiny levels is important to furthering our understanding of how gravity works at the quantum scale. The smaller the force, the more likely researchers are to run into the Heisenberg uncertainty principle, where the act observation effects the precision of the result observed through what's called "quantum back-action." This limitation of accuracy is called the Standard Quantum Limit (SQL) – a limit which scientists have spent decades trying to get closer to by minimizing quantum back-action in the ultra-sensitive mechanical oscillators which they use to detect force.

In this case the mechanical oscillator consisting of a gas of just 1200 rubidium atoms trapped within a light field and chilled to almost absolute zero. By modulating the amplitude of this light field, scientist are able to apply force which is then measured by a probe beam.

"When we apply an external force to our oscillator it is like hitting a pendulum with a bat then measuring the reaction," adds Sydney Schreppler, the lead author of the paper. "A key to our sensitivity and approaching the SQL is our ability to decouple the rubidium atoms from their environment and maintain their cold temperature. The laser light we use to trap our atoms isolates them from external environmental noise but does not heat them, so they can remain cold and still enough to allow us to approach the limits of sensitivity when we apply a force."

The experiment resulted in the most sensitive measurement to date, at a factor of only four above the SQL – all previous measurements were factors of six or eight of magnitude.

Schreppler believes it is possible to achieve even more accurate measurements by using still colder atoms and more efficient optical detection.

“A scientific paper in 1980 predicted that the SQL might be reached within five years,” she points out. “It took about 30 years longer than predicted, but we now have an experimental set-up capable both of reaching very close to the SQL and of showing the onset of different kinds of obscuring noise away from that SQL.”